DC-DC converters have been described in detail in e.g. “Daniel M. Mitchell, DC-DC Stitching Regulator Analysis, McGraw Hill 1988”.
The converters are commercially available, e.g. from HN Electronic Components GmbH, Langenselbold, and the function thereof is readily explained. Furthermore DC-DC converters are available from Technoproject, as described in “http://www.techno-project.de”. Presently DC-DC converters are used for many applications, e.g. in combination with electric charging apparatuses for mobile phones.
Energy management is one of the fundamental problems in a project of portable, electronic devices. This problem pertains all the elements that make a device portable and affects service, time of utilizability, size and weight of the same.
Conventional charging apparatuses employ linear regulators, where a power transistor is integrated in the charging apparatus and the regulator requires a regulated external AC-adapter having a very accurate output voltage such that the voltage drop across the power transistor is reduced and the power dissipation is maintained within acceptable values.
A major disadvantage of commonly used AC-adapters (AC=alternating current, connector) is the high cost. In many applications unregulated AC-adapters are widely used whereby AC-adapters contain DC-DC-converters, such that the output voltage is continuous.
A further disadvantage of unregulated adapters is that they do not have a feed-back loop maintaining the output voltage constant resulting in a much greater load-dependant output voltage.
Another disadvantage is that the power dissipation is high such that it is in many cases not possible to integrate the power transistor.
This leads to the further disadvantage, that the power transistors has to be installed externally and a charging current has to be reduced.
Another version of an AC-adapter consists of a small transformer with a diode bridge that produces a half wave rectified waveform. The diode bridge is followed by a PTC resistor (positive temperature coefficient), which is installed in order to limit the maximum current towards a load, e.g. a mobile phone, whereby the mobile phone contains an integrated power transistor, which is connected in a diode configuration. The major disadvantage of this design is that the charging time is considerably increased.
Another state-of-the-art arrangement is a DC-DC converter, which has a very high efficiency and which can be used with both regulated and unregulated DC-DC converter-AC-adapters. An advantage of this configuration is, that it has an internal current limitation permitting a direct connection of the mobile phone to a car battery. Due to the high efficiency the power transistor can be integrated. A major disadvantage of this configuration is the cost and size of an inductance coil necessary for this setup.
A further conventional setup uses pulsed charging requiring an external AC-adapter, which must contain a DC-DC converter with current limitation. In this configuration the power transistor functions as a charging switch at very low frequencies, e.g. in the range of 0.1-10 Hz. The major disadvantage of this configuration is, that there is a possibility, that voltage peaks exceed the maximum voltage defined for e.g. Lithium Ion batteries by the production process. Nevertheless this type of charging is highly efficient permitting the power transistor to be integrated. The configuration requires no external components and can be adapted to a large variety of adapters.
One major disadvantage of the pulsed charging configuration is that this design requires a change in a circuit configuration in order to adapt a specific AC-adapter. A further disadvantage is that an external diode is required. Yet a further disadvantage is, that battery producers have not yet given an “OK” for using voltages greater that the one specified.